Starting with mockups as markup makes it hard for me to be creative. It binds my brain immediately to the implementation aspect of the design, rather than freely exploring creative avenues.

Being able to nudge and push things around in Photoshop (or Fireworks, my preferred program) is invaluable. This takes away the overhead of thinking about browser related implementation issues. Don't underestimate this overhead. I find that all my designs start looking the same if I start with markup, in contrast to the more varied and interesting designs that I come up with when starting in Fireworks.